Nonuniform pulse sonar navigation system

ABSTRACT

A nonuniform pulse sonar system employing Doppler information for navigational purposes is disclosed. A train of pulses including a plurality of energy pulses having varying predetermined widths and spacing is provided by a nonuniform pulse generator to energize a transducer which serves as a projector adapted to transmit acoustical energy into a body of water. Reflected energy is sensed by the transducer, which also serves as a hydrophone, and subsequently processed by a tracker which selects and reproduces, as a continuous frequency signal, the predominant frequency in the frequency spectrum of the reflected energy detected. A demodulator unit is employed to compare the continuous frequency signal with the frequency of the energy pulses used to energize the transducer, the frequency difference constituting a Doppler frequency that is proportional to the velocity of a craft, on which the sonar system is mounted, relative to the object, or objects, such as the ocean floor, from which the transmitted energy is reflected.

United States Patent I1113,617,995

[ Inventor Thomas Primary ExaminerRichard A. Farley Reseda, Calif.Attorney-Robert E. Geauque [21] Appl. No. 814,987 [22] Filed Apr. 10,1969 [45] Patented Nov. 2, 1971 ABSTRACT: A nonuniform pulse sonarsystem employing [73] Assignee The Marquardt Corporation Dopplerinformation for navigational purposes isdisclosed. A Van Nuys, Calif.train of pulses including a plurality of energy pulses having varyingpredetermined widths and spacing is provided by a nonuniform pulsegenerator to energize a transducer which [54] NONUNIFORM PULSE SONARNAVIGATION serves as a projector adapted to transmit acoustical energySYSTEM into a body of water. Reflected energy is sensed by thetransloclaims, 5 Drawing Figs ducer, which also serves as a hydrophone,and subsequently processed by a tracker which selects and reproduces, asa con- [52] U.S.Cl. 340/3 R, tinuous frequency signal, the predominantfrequency in the 340/3 343/8 frequency spectrum of the reflected energydetected. A

[51] Int. Cl. G015 9/66 demodmator unit is employed to compare thecontinuous [50] Fleld of Search 340/3, 3 D; frequency Signal with thefrequency of the enel-gy pulses used 343/8 to energize the transducer,the frequency difference constitutin a Do pler frequency that isproportional to the velocity of [56] Reta-em. cued a raft, O; which thesonar system is mounted, relative to the UNITED STATES PATENTS object,or objects, such as the ocean floor, from which the 3,121,856 2/1964Finney 340/3 transmitted energy is reflected.

44 Fore Ansm Port Non Uniform Truuaminei PUISB Genera tor ntexe DuplexerDuptexe PATENTED Nnv2 IQYI SHEET 2 OF 2 A T i T W I W *1 zoI I I I I 120120 c 1 I w I20 I so I F I H H IzoI I00 I so so I20I 22o Flg. 5.

Pulse A B D One Generator 2 Dewy 2 Shot E IOO F lg. 4.

84 82 --1 Gate Generaior 32 Time AGC 7 Vorymg 76 Gain Driver I From AGCWBuffe Detector Thomas A. Goule'r,

38 |N\B/$NTOR.

Fig. 5.

ATTORNEY.

BACKGROUND OF THE INVENTION This invention relates to Dopplernavigational apparatus and more particularly to a novel sonar navigationsystem for determining the velocity of a marine vehicle or craftrelative to the floor or bottom of a body of water.

Several Doppler navigation systems are currently in use on military andcivilian aircraft and marine vehicles. These systems include variousradar and sonar devices which involve the well-known Doppler phenomenonwherein the Doppler frequency shift of received signals, relative to thefrequency of transmitted signals that are reflected by terrain below thecraft or vehicle, is detected. Among these Doppler navigation systemsare sonar systems which include transmitting and receiving equipmentadapted to detect the Doppler frequency shift of sound waves reflectedby the ocean bottom. The principles of operation in the radar and sonarsystems are generally the same, the structural differences in theapparatus employed being primarily necessitated by differences infrequency, propagation velocity, craft velocity, and techniques fordetecting and transmitting.

Generally, Doppler sonar systems that are available in the prior artemploy a continuous wave sonar Doppler device that will accept allinformation resulting from the transmitted signals. Since theinformation is at a continuous rate, it is not possible to timediscriminate in an effort to separate Doppler frequencies between fixedand moving objects. Additionally, a crosstalk between the transmitterand receiver is a problem in continuous wave systems, as well as theadverse signal effects derived because of volume reverberation. Byvolume reverberation, it is meant that sea water, in contrast to freshwater, contains millions of micro-organisms in a more or less uniformmanner throughout temperate and tropic regions of the world. Thesemicro-organisms, and the gas bubbles they create, produce aback-scattering phenomenon which is known to those skilled in the sonarart as volume reverberation.

Among some of the prior art devices employing continuous wavetransmission which suffer from the above difficulties and problems arethose represented in the disclosures of U.S. Pat. Nos. 2,912,671;2,961,190; 3,065,463; and 3,231,852. All of these systems employ acontinuous wave transmission technique in which crosstalk is a problem,the necessity of both a transmitter and a receiver, and the inability ofthe con tinuous wave transmission system to accurately navigate fromvolume reverberation signals in a disturbed water environment.

Pulse Doppler principles, in a general form, have been employed in radardevices to detect and monitor relative motion. However, these deviceshave employed a pulse system for purposes other than those involved inthe present invention. For example, pulse techniques have been employedin radar systems to increase the ratio of peak power to average powerabove unity to produce greater range, to reduce power consumption, andto allow a single antenna to be employed for transmission as well as forreception by time sharing the antenna. Inasmuch as the medium throughwhich energy is propagated in sonar systems is water, these reasons arenot applicable for sonar navigational purposes.

To the extent that pulse Doppler techniques have been employed withsonar systems, a unifonn pulse train has generally been used. In the useof a uniform pulse train, however, the pulsed transmission, andconsequently the pulsed return signal, contains frequency sidebands inaddition to the transmitted frequency. These frequency sidebands aregenerally constant and of sufficient power level to produce navigationalerrors as a result of the tracking apparatus incorrectly locking onto asideband frequency.

These navigational errors can be eliminated if the power level of thesidebands is reduced and the sideband frequencies are caused to varyinstead of being allowed to remain constant. This can be accomplished bytransmitting a train of pulses of varying width and spacing, and it isthis technique that is employed in the present invention.

SUMMARY OF THE INVENTION Briefly described, the present inventioninvolves a pulse sonar system that is free from the problems anddifiiculties encountered with prior art systems.

More particularly, a pulse sonar system adapted to project a train ofpulses including pulses of varying width and spacing is provided.Projected energy that is reflected by the ocean floor as well as otherobjects in the path of propagation is sensed by an appropriatetransducer and then processed by a tracker. A demodulator unit isemployed to determine the frequency difference between the transmittedand received signals. This frequency difference, which constitutes aDoppler frequency, is proportional to the velocity of the craft bearingthe sonar system relative to the ocean floor.

It is therefore an object of the present invention to provide animproved pulsed sonar navigational system for determining the relativevelocity of a marine vehicle.

A further object of the present invention is to provide a pulsed sonarnavigational system that is immuned to navigational errors caused byreceived sideband signals.

Another object of the present invention is to provide a pulsed sonarsystem in which a nonuniform pulsed train is transmitted to effect areduction in the power level of sideband frequency signals.

DESCRIPTION OF THE DRAWINGS Other objects, and many of the attendantadvantages of this invention will be more readily appreciated as thesame becomes better understood by reference to the following detaileddescription, which is to be considered in connection with theaccompanying drawings in which like reference symbols designate likeparts throughout the figures thereof and wherein:

FIG. 1 is a schematic illustration of an exemplary configuration of anacoustical transducer array which is mounted on the hull of a marinevehicle;

FIG. 2 is a general block diagram illustrating a pulsed sonar system inaccordance with the present invention;

FIG. 3 is a block diagram illustrating an exemplary gain control circuitthat may be employed in the system illustrated by FIG. 2;

FIG. 4 is a block diagram illustrating an exemplary nonuniform pulsedgenerator that may be included in the system illustrated by FIG. 2; and

FIG. 5 is a schematic diagram illustrating waveforms that are useful inexplaining the operation of the nonuniform pulsed generator illustratedby FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a marinevehicle 10 may be employed to carry a pulsed sonar navigational system.For purposes of explanation the fore-aft axis of the vehicle 10 has beendesignated the y-axis and the port-starboard axis has been designatedthe x-axis, as illustrated. A plurality of acoustical transducers 12,l4, l6 and 18 may be mounted on the hull of the vehicle 10 along thexand y-axes in a noncritical fixed spaced-apart relationship. Theacoustical transducers 12, 14, 16 and 18 y-axis preferably highlydirectional transducers which may be used for both projection ofacoustical signals and for sensing acoustical signals and may be any ofthe various types of transducers well known in the prior art. Ifdesired,the transducers may be arranged in a cluster outwardly extending fromthe side of the craft 10 instead of on the bottom of the craft 10 asillustrated. For present purposes it is sufficient to note that thetransducers produce, when operating in the propagation mode, narrowbeams of ultrasonic energy which are backscattered or reflected from theocean floor. In an operative embodiment the beams were radiated at anangle of approximately 30 with respect to a vertical line extendingthrough the vehicle 10 and with a width of approximately 3. It is to benoted that separate transducers may be employed for the purpose oftransmitting and of receiving.

Consistent with the present invention is the fact that the floor of theocean, or of any body ofwater, generally is not perfectly flat and willtherefore reflect impinging ultrasonic energy in all directions suchthat at least some of the energy projected by the respective transducers12, 14, 16 and 13 will be reflected in the direction of the transducers.It will become more apparent that the use of two transducers along eachaxis,

" forexample, transducers l2 and 16 along the Y-axis serves tocompensate for frequency changes caused by roll, pitch or yaw of thevehicle 10.

Referring to FIG. 2, a transmitter 20 applies radio energy to theacoustical transducers 12, 14, 16 and 18 through the respectiveduplexers 22, 24, 26 and 28. A master oscillator 30 may be employed toprovide RF energy to the transmitter 20. r

In an operative embodiment, 300 kHz. has been found to be a suitablefrequency. Blanking signals for appropriately turning the transmitter onand off to produce the desired series of pul ses to be projected by thetransducers 12, 14, 16 and 18 are provided by a nonuniform pulsegenerator 32.

As earlier mentioned, navigational errors can be eliminated or otherwisereduced if the power level and constancy of received sidelobefrequencies is reduced. These sidelobe frequencies are produced as aresult of the constant pulse repetition frequency of uniform pulsetrains employed in prior art pulsed sonar systems. It has been foundthat if a nonuniform pulse train is employed in the transmission ofultrasonic signals, the same sidelobe frequencies are not con stantlyreceived. Further, those sidelobe frequencies that are received have apower level that is significantly less than the power level of thecentral frequency that is to be tracked. The result is that the trackercircuitry, employed in sonar systems, will more easily fix on thecentral frequency and ignore the sidelobe frequencies, resultingnavigational errors being thereby eliminated and/or reduced.

The projected or transmitted pulsed signals are directed in thedirection of the ocean floor and are subsequently backscattered orreflected in the direction of the acoustical transducers 12, 14, 16 and18 which serve, when operating in a receive mode, to convert acousticalenergy to electrical energy. It is to be understood that each of thetransducers 12, 14, 16 and 18 will, generally speaking, sense or receiveonly that reflected energy that was originally projected by the sametransducer. For example, transducer 12 will sense only the reflectedenergy that was originally projected by transducer 12.

The electrical energy provided by each of the transducers is appliedthrough the respective duplexers 22, 24, 26 and 28 to a receivercircuit. For purposes of explanation only the receiver circuitryassociated with the duplexer 28 has been illustrated in FIG. 2 althoughit is to be understood that a sonar system, in accordance with thepresent invention, would include a receiver circuit for each of therespective transducers employed.

Received signals are applied through a preamplifier 34, a gating circuit36, and an amplifier 38 to a gain control circuit 40, a tracker circuit42 and an automatic acquisition circuit 44. The gain control circuit 40,which will be described hereinafter in connection with FIG, 3, producesan output signal which is applied as a feedback signal over the lead 44to the preamplifier circuit 34.

Protection of the receiver circuit is provided by reducing the gain ofreturn signals. This is accomplished by applying the nonuniform pulsetrain provided by the pulse generator 32 to the gating circuit 36, andto a gating circuit in the gain control 40. The gating circuits serve toblock the passage of signals during the period in which pulses are beingapplied thereto.

The tracker circuit 42 generally serves to provide a continuous signalhaving a frequency the same as that of the received pulse signals. Thetracker circuit 42 is commonly referred to by those skilled in the artas a phase lock loop which locks upon itselfin terms of both frequencyand phase. A phase lock loop would generally include a voltagecontrolled oscillator and a control circuit which is a combination of anamplifier and an integrator. Any of the phase lock loop circuits wellknown in the prior art may be employed in the tracker circuit 42 toaccomplish the desired purpose. Within the tracker 42, a comparison ismade between the voltage controlled oscillator output and the pulsedincoming signal, a difference signal being produced which presents adifference not only in frequency but in phase. This latter signal isthen processed by the VCO control circuit which steers the voltagecontrolled oscillator to the phase and frequency that is necessary toattain exact synchronization with the incoming signal frequency from theamplifier 38. It is to be kept in mind that the reference frequency ofthe voltage controlled oscillator is a continuous frequency and thefrequency of the incoming signal. from the amplifier 38 is pulsedinformation. As a result, the tracker 42 is intended to be in operationonly during the period in which the signal from the amplifier 38 isreceived. For this purpose the difference signals generated by thetracker 42 are applied to the acquisition circuit 44 over a lead 46while received signals are applied through the preamplifier 34, thegatingcircuit 36, and the amplifier38 to the automatic acquisitioncircuit 44 by way of a lead 48. The output of the automatic acquisitioncircuit is applied back to the track circuit 42 overa lead 50. Theautomatic acquisition circuit 44 serves tosychronize thevoltagecontrolled oscillator signals with the frequency of the receivedsignals.

In general, the automatic acquisition circuit 44 includes adiscriminator, which is'a part of thephase lock loop in the tracker 42,and a shorting switch coupled between the integrator of the VCO controlcircuit and the discriminator such that when the switch is shorted orclosed, the VCO control circuit will hold and control the voltagecontrolled oscillator in such a manner that it will not change frequencybut will continue to produce the frequency as of the last informationreceived.

The acquisition circuit may employ logic information from a phasedetector and a coherent detector to control the switch for shorting outthe signal appearing over the lead 50. This results in the integratoroutput effecting a change in the frequency of the voltage controloscillator in the direction of the frequency of received signals suchthat frequency coincidence, and as a consequence frequency lock, canoccur. An exemplary automatic acquisition circuit is described in detailin a copending U.S. Pat. application, Ser. No. 737,123, filed June 14,1968, by Thomas A. Goulet and Irving A. Sofen, entitled Pulse SonarNavigational System" and assigned to the same assignee as the instantinvention.

The output of the tracker circuit 42 is also applied to a demodulatorunit 52 which is coupled to the master oscillator 30. The demodulatorunit 31 serves to compare the originally transmitted frequency providedby the oscillator 30 with the frequency of the continuous signalprovided by the voltage controlled oscillator in the tracker circuit 42.The result of this comparison is a frequency difference whichconstitutes the Doppler frequency caused by the relative movementbetween the transducers mounted on the hull of the marine vehicle 10 andthe ocean floor.

The demodulator unit 52 is adapted to provide a pair of output signals,one over each of two channels, the signals being respectively a positiveDoppler signal and a negative Doppler signal. These outputs aregenerated in a fashion such that they will not occur simultaneously butinstead in sequence. In operation, the signal generated by the masteroscillator 30 is supplied to each of the two channels of the demodulatorunit 52. The oscillator frequency signal is phase shifted by degreesbefore being applied to one channel and applied directly to the otherchannel such that both 0 and 90 phases are used for demodulatingpurposes. The frequency of the continuous signal provided by the tracker42 is compared to this oscillator frequency in each of the separatedemodulator channels. if the Doppler frequency is positive, indicating aclosing rate, an output signal will first occur on the positivedemodulator lead 54. If the Doppler frequency is negative, indicating adown or opening rate, an output signal will first occur on the negativedemodulator lead 56.

The outputs of the demodulator 52 are applied to a translator 58. It isto be noted that in addition to the output shown from the demodulatorunit 52, three additional pairs of demodulator output leads are shown asindicated by the numerals 60, 62 and 64 which are respectively connectedto circuitry similar to that just described in connection with thetransducer 18.

After the signals are applied to the translator, the signals from thevarious channels of the system occupy sequential time channels andtherefore do not occur simultaneously. The signals are then processed soas to then be digitally added in an adder 66 which serves to sum thesignals from the various outputs of the translator 58 in such a fashionthat the resulting signals represent four velocities which may bedisplayed over a plurality of suitable indicator devices 68, 70, 72 and74, which velocities are respectively indicative of fore-velocity,aftvelocity, starboard-velocity and port-velocity.

Referring to FIG. 3, an exemplary gain control circuit 40 isillustrated. More particularly received signals are applied from theamplifier 38 (FIG. 2) through a buffer circuit 76 to an automatic gaincontrol detector circuit 78. The signals provided by the automatic gaincontrol detector circuit 78 are applied to a time varying gain circuit80. Signals are also applied to the time varying gain circuit 80 throughan amplifier 82 and a gate 84, coupled in series, which gate 84 iscoupled to the nonunifonn pulse generator 32 (FIG. 2). The gate 84,which receives pulses from the nonunifonn pulse generator 32, serves toreduce the gain of the receiver during the transmit period. The timevarying gain circuit 80, after the period that the gain has been reducedby the gate 84, is adapted to effect normal gain according to a presettime constant.

The time varying gain control circuit 80 may include, for example, an RCnetwork for establishing the decay of gain as it reverts to normal gainaccording to the time constant provided for by the RC network.

The automatic gain control loop formed by the buffer 76, the AGCdetector 78, the time varying gain circuit 80, the AGC driver 86, andthe preamplifier 34 serves to control the overall gain of return signalsto be processed by regulating amplification of the preamplifier 34. Asthe preamplifier 34 is driven more positive by signals from the AGCdriver 86, the gain of the signal is reduced. Conversely, when thepreamplifier 34 is driven more negative the signal gain is increasedproportionately.

An exemplary nonuniform pulse generator circuit 32 is i]- lustrated inFIG. 4. A pulse generator 88 is provided to supply a uniform train ofpulses each having a 20 microsecond psec. duration, or pulse width,wherein each pulse is separated by a 100 p.886. spacing as illustratedin FIG. 5A. This train of pulses is applied to an AND gate 90 as oneinput thereto and to a divider circuit 92 which may be a multivibratorcircuit of a type well known in the prior art. The divider circuit 92serves to provide a positive pulse corresponding to every other pulseapplied thereto from the pulse generator 88. The pulse train provided bythe divider circuit 92, as illustrated by FIG. 5B, is applied as thesecond input to the AND gate 90. The AND gate 90 will provide a pulsehaving a 20 uses. duration or pulse width whenever there is coincidencebetween the signals applied from the pulse generator 88 and the dividercircuit 92. The pulse train provided by the AND gate 90, as illustratedin FIG. 5C, is applied to an OR gate 92.

The pulse train provided by the divider 92 is applied through a delayline 94 to a second divider circuit 96. The delay is of a duration tocause the pulses provided by the divider 92 to be delayed a length oftime equivalent to the duration between the leading edges of the pulsesprovided by pulse generator 88. In the present example, the delay 94would be equal to 120 psec. The delayed pulse train provided by thedivider 96, illustrated by FIG. 5D, is applied to a one-shot circuit 98which serves to provide a 60 psec. pulse whenever a positive pulse fromthe divider 96 is applied thereto. This 60 psec. pulse is applied to theOR gate 92 as a second input thereto. The pulse train provided by theone-shot circuit 98 is illustrated by FIG. 5E. The output of the OR gate92, illustrated by FIG. 5F, will be a nonuniform pulse train comprising,in sequence, a 20 #sec. pulse followed by a 100 usec. spacing, a 60psec. pulse followed by a 60 psec. spacing, and a 20 usec. pulsefollowed by a 220 psec. spacing. Identical cycles of these three pulseswith the described spacings will then follow to form the desirednonuniform pulse train which is, as earlier explained, applied to thetransmitter 20, the gate 36, and the gain control circuit 40.

While a preferred embodiment of the present invention has been describedhereinabove, it is intended that all matter contained in the abovedescription and shown in the accompanying drawings be interpreted asillustrative and not in a limiting sense and that all modifications,constructions and arrangements which fall within the scope and spirit ofthe present invention may be made.

What is claimed is:

l. A pulsed sonar system for use with a marine vehicle, said sonarsystem comprising:

projector means for projecting a train of nonuniform pulsed acousticalsignals into a body of water, said projector means including anoscillator for providing a master signal having a selected frequency;

receiver means for generating a train of pulsed electrical signalsrepresenting reflected portions of said pulsed acoustical signals;tracking means responsive to said pulsed electrical signals forgenerating a continuous signal having a frequency corresponding to thefrequency of said electrical signals;

demodulator means for comparing the frequency of said continuous signalwith the frequency of said master signal to develop a differencefrequency signal; and

means for processing said difference frequency signal to determine thevelocity of said marine vehicle.

2. The sonar system defined by claim 1 wherein said pro jector meansfurther includes:

a transmitter, operatively coupled to said oscillator, for

generating pulsed transmit signals;

nonuniform pulse generator means for controlling the width and spacingof successive ones of said pulsed transmit signals; and

acoustical transducer means, operatively coupled to said transmitter forconverting said pulsed transmit signals into pulsed acoustical signals.

3. The sonar system defined by claim 2 wherein said nonuniform pulsegenerator means comprises:

first means for generating a train of pulses having a uniform pulsewidth and spacing;

second means for blanking alternate ones of said pulses in said train ofpulses; and

third means, operatively coupled to said second means, for

replacing alternate ones of the blanked pulses with a pulse having apulse width not the same as said uniform pulse width.

4. The sonar system defined by claim 2 wherein said acousticaltransducer means comprises a plurality of acoustical transducersarranged to project and sense acoustical signals in a plurality ofpredetermined directions.

5. The sonar system defined by claim 1 wherein said receiver meansincludes:

a gain control circuit; and

means for causing said gain control circuit to reduce the gain of saidreceiver means for selected periods during which said nonuniform pulsedacoustical signals are being propagated.

6. The sonar system defined by claim 1 wherein said tracking meansincludes a phase lock loop.

7. The sonar system defined by claim I wherein said difference frequencysignal represents a Doppler shift frequency that is proportional to thevelocity of said marine vehicle.

8. In a pulsed sonar system including a transmitter for generatingpulsed acoustical signals in response to electrical signals having aselected master frequency, a receiver for providing pulsed electricalsignals in response to reflected acoustic signals, and demodulator meansfor providing a Doppler shift frequency proportional to the velocity ofa marine vehicle by comparing the frequency of said pulsed electricalsignals to said master frequency, the improvement comprising nonuniformpulse generator means, operatively coupled to said transmitter, forcontrolling the pulse width and the pulse spacing of successive ones ofsaid pulsed acoustical signals to be nonuniform in pulse width and pulsespacing.

second means for effectively eliminating every other successive one ofsaid pulses in said train of pulses; and third means for replacing everyother successive one of the effectively eliminated pulses with a pulsehaving a predetermined pulse width not the same as said uniform pulsewidth. 10. The apparatus defined by claim 8 wherein said nonuniformpulse generator means is further operatively coupled to 9 The apparatusdefined by claim 8 wherein said nnu 10 said receiver for reducing thegain of said receiver for selected nifonn pulse generator meanscomprises:

first means for generating a train of pulses having a uniform pulsewidth and pulse spacing;

periods of time during which said pulsed acoustical signals are beinggenerated.

I I! i i I

1. A pulsed sonar system for use with a marine vehicle, said sonarsystem comprising: projector means for projecting a train of nonuniformpulsed acoustical signals into a body of water, said projector meansincluding an oscillator for providing a master signal having a selectedfrequency; receiver means for generating a train of pulsed electRicalsignals representing reflected portions of said pulsed acousticalsignals; tracking means responsive to said pulsed electrical signals forgenerating a continuous signal having a frequency corresponding to thefrequency of said electrical signals; demodulator means for comparingthe frequency of said continuous signal with the frequency of saidmaster signal to develop a difference frequency signal; and means forprocessing said difference frequency signal to determine the velocity ofsaid marine vehicle.
 2. The sonar system defined by claim 1 wherein saidprojector means further includes: a transmitter, operatively coupled tosaid oscillator, for generating pulsed transmit signals; nonuniformpulse generator means for controlling the width and spacing ofsuccessive ones of said pulsed transmit signals; and acousticaltransducer means, operatively coupled to said transmitter for convertingsaid pulsed transmit signals into pulsed acoustical signals.
 3. Thesonar system defined by claim 2 wherein said nonuniform pulse generatormeans comprises: first means for generating a train of pulses having auniform pulse width and spacing; second means for blanking alternateones of said pulses in said train of pulses; and third means,operatively coupled to said second means, for replacing alternate onesof the blanked pulses with a pulse having a pulse width not the same assaid uniform pulse width.
 4. The sonar system defined by claim 2 whereinsaid acoustical transducer means comprises a plurality of acousticaltransducers arranged to project and sense acoustical signals in aplurality of predetermined directions.
 5. The sonar system defined byclaim 1 wherein said receiver means includes: a gain control circuit;and means for causing said gain control circuit to reduce the gain ofsaid receiver means for selected periods during which said nonuniformpulsed acoustical signals are being propagated.
 6. The sonar systemdefined by claim 1 wherein said tracking means includes a phase lockloop.
 7. The sonar system defined by claim 1 wherein said differencefrequency signal represents a Doppler shift frequency that isproportional to the velocity of said marine vehicle.
 8. In a pulsedsonar system including a transmitter for generating pulsed acousticalsignals in response to electrical signals having a selected masterfrequency, a receiver for providing pulsed electrical signals inresponse to reflected acoustic signals, and demodulator means forproviding a Doppler shift frequency proportional to the velocity of amarine vehicle by comparing the frequency of said pulsed electricalsignals to said master frequency, the improvement comprising nonuniformpulse generator means, operatively coupled to said transmitter, forcontrolling the pulse width and the pulse spacing of successive ones ofsaid pulsed acoustical signals to be nonuniform in pulse width and pulsespacing.
 9. The apparatus defined by claim 8 wherein said nonuniformpulse generator means comprises: first means for generating a train ofpulses having a uniform pulse width and pulse spacing; second means foreffectively eliminating every other successive one of said pulses insaid train of pulses; and third means for replacing every othersuccessive one of the effectively eliminated pulses with a pulse havinga predetermined pulse width not the same as said uniform pulse width.10. The apparatus defined by claim 8 wherein said nonuniform pulsegenerator means is further operatively coupled to said receiver forreducing the gain of said receiver for selected periods of time duringwhich said pulsed acoustical signals are being generated.